Another defendant in the case, veteran pro-democracy campaigner Koo Sze-yiu, was jailed for five months, after he was found guilty of unauthorised assembly following a trial.
Hundreds of people had joined a march on Hong Kong Island on October 5, 2019, to demonstrate against a ban on wearing masks that came into force that day.
In sentencing, Eastern Court magistrate Daniel Tang, described Wong, 24, as an iconic figure, noting that he had given media interviews on the day in question.
Tang said the four-month prison sentence was to be served separately from the 13-and-a-half month jail term Wong is already serving over a siege of police headquarters in 2019.
Wong is also one of the 47 pro-democracy figures charged with subversion under the national security law for joining primary polls last year which were aimed at selecting candidates for Legco elections that never happened in the end.
Koo, 75, only left prison last week after serving time for desecrating the national flag during a demonstration in July last year. He has stage four cancer.